This invention pertains to a method of detecting and correcting air-fuel ratio or torque imbalances in individual cylinders of a three-cylinder engine or banks of three cylinders in a V6 engine using a single sensor. More specifically, this invention pertains to the use of a frequency-domain characterization of the pattern of such imbalances in detecting and correcting them.
There is a continuing need for further refinement of air-fuel ratio (A/F) control in vehicular internal combustion engines. At present, A/F is managed by a powertrain control module (PCM) onboard the vehicle. The PCM is suitably programmed to operate in response to driver-initiated throttle and transmission gear lever position inputs and many sensors that supply important powertrain operating parameters. The PCM comprises a digital computer with appropriate processing memory and input-output devices and the like to manage engine fueling and ignition operations, automatic transmission shift operations and other vehicle functions. In the case of such engine operations, the computer receives signals from a number of sensors such as a crankshaft position sensor, and an exhaust oxygen sensor.
Under warmed-up engine operating conditions, the PCM works in a closed loop continuous feedback mode using the voltage signals from an oxygen sensor related to the oxygen content of the exhaust. The crankshaft angular position information from the crankshaft sensor and inputs from other sensors are used to manage timing and duration of fuel injector duty cycles. Zirconia-based, solid electrolyte oxygen sensors have been used for many years with PCMs for closed loop computer control of fuel injectors in applying gasoline to the cylinders of the engine in amounts near stoichiometric A/F. The PCM is programmed for engine operation near the stoichiometric A/F for the best performance of the three-way catalytic converter.
With more strict emission standards gradually phasing in, there is a need for further refinement of automotive technologies for emissions reduction. One such refinement is the use of a linear response (wide-range) A/F sensor in the exhaust pipe(s) in place of the current zirconia switching (nonlinear) oxygen sensor. Experiments have demonstrated that significant reductions in tailpipe NOx emissions are possible because of the more precise A/F control offered by a linear A/F sensor.
A second refinement is to increase vehicle fuel economy by diluting the air-fuel mixture with excess air (lean burn) or with exhaust gas recirculation (external EGR). The maximum benefit is achieved at the highest dilute limit. However, in a multi-cylinder engine, the limit is constrained by development of partial burns and possibility of misfire in the cylinder(s) containing the leanest mixture. This happens due to maldistribution of air, fuel or EGR in different cylinders. Thus, a new capability for the control of every cylinder air-fuel ratio by software is needed. Here, the intention would be to control only one variable (e.g., air, fuel or spark) to create uniform A/F or torque in all cylinders since only a single variable (e.g., A/F, O2 or torque) would be measured. Clearly, single-loop feedback controllers around various sensors can operate independently to control air, fuel or spark in every cylinder.
Another motivation for all-cylinder A/F control is cost containment. For very low emission applications, fuel injectors of high precision (i.e., very small tolerances of less than 3%) are thought to be required. Achievement of this degree of tolerance, if possible at all, would be costly. A better solution would be to have a software means to compensate for the differences between fuel injectors in real-time operation of the engine. Another source of cylinder imbalances in a multi-cylinder engine is the inherent engine maldistribution due to variable breathing capacities into various cylinders. The air maldistribution can result in A/F or torque imbalances for which a software solution is sought.
Accordingly, it is seen that new emission reduction strategies for automotive gasoline engines would be enabled or enhanced by the development of a process for detecting and correcting fuel, air or spark imbalances between cylinders of a multi-cylinder engine.
In this invention, a process is provided that would balance A/F or torque amongst all cylinders of a three-cylinder engine or separately in either bank of a V6 engine. The benefits in terms of emissions reduction, fuel economy and driveability will depend on the degree of A/F or torque imbalances present in the engine and is engine dependent. In general, it is estimated that the benefit would depend on exhaust system configuration as well. For example, the benefit in a V6 engine with dual banks of unequal pipe lengths is larger when a single sensor is used for control and when fuel injectors have larger tolerances.
A principal cause, but not necessarily the sole cause, of cylinder A/F imbalances in a fuel-injected engine is differences in the delivery rates of the fuel injectors. Fuel injectors are intricate, precision-made devices, but the delivery rates of xe2x80x9cidenticalxe2x80x9d injectors may vary by as much as xc2x15%. Thus, the normal operation of a set of such injectors may be expected to lead to the delivery of varying amounts of fuel in the respective cylinders even when the PCM specifies identical xe2x80x9cinjector onxe2x80x9d times. If the air flow rate or the exhaust gas recirculation rate is not varying in proportion with the fuel imbalances, there can be significant differences in A/F and/or torque among cylinders.
In a three-cylinder (or dual exhaust system V6) engine, individual cylinder maldistributions of air, fuel and EGR cause fluctuations in the instantaneous oxygen sensor voltages measured downstream at the point of confluence in the exhaust manifold. These O2 sensor voltages are representative of the A/F of the cylinders. The actual A/F signal is periodic with the successive exhausts of the three cylinders, but the periodic pattern remains similar over prolonged engine operation especially if the pattern is due mainly to variances in fuel injector deliveries. Any arbitrary pattern of cylinder to cylinder differences in A/F ratio can be represented by a combination of simpler basic A/F patterns here referred to as xe2x80x9ctemplatesxe2x80x9d. In this notation, a template consists of a unique pattern of xe2x88x921, 0 and +1 units of A/F or a multiple thereof in each cylinder only. Negative and positive signs imply fuel-rich and fuel-lean A/F, respectively, and 0 implies stoichiometric A/F for a particular cylinder exhaust event. At this point the values of xe2x88x921 and +1 simply indicate rich and lean A/F without regard to the magnitude of the departure of the ratio from the stoichiometric value, typically about 14.7 for most common gasoline fuels available today.
Obviously, each cylinder could experience a rich or lean A/F when the PCM is trying to control the overall A/F at the stoichiometric ratio. However, it has been determined in connection with this invention that the patterns of all possibilities are not independent of each other. It turns out that the number of independent basic patterns in this representation is equal to the number of cylinders. Specifically for a three-cylinder engine, any unknown pattern of imbalances can be reduced to a combination of three basic patterns T1, T2 and T3 shown in FIG. 1. Referring to FIG. 1, template T1 has the pattern +1, 0, xe2x88x921 (i.e., lean A/F, stoichiometric A/F and rich A/F) for cylinders 1, 2, 3 respectively. Template T2 is the pattern xe2x88x921, +1, 0 and template T3 is the pattern +1, +1, +1.
It has been further discovered in connection with this invention that the pattern of unknown three cylinder A/F imbalances with magnitudes (a, b, c) can be uniquely related to the above three templates by appropriate weighting factors (f1, f2, f3) applied to the values of the terms of each template (FIG. 1). Thus, the knowledge of the set of coefficients (f1, f2, f3) is equivalent to knowledge of the unknown values of the imbalances (a, b, c) in the engine""s three cylinders. The coefficients may have positive or negative values or the value of zero. Often it is preferred that the coefficients have values expressed as percentages of the cylinder weighting factors of the templates.
It also turns out that that pattern of T3, identically rich or lean in all cylinders, is corrected by normal feedback closed-loop operation of the current O2 sensor and the PCM. Therefore, this template does not need to be used in detecting imbalances a, b and c. As will be shown, the total imbalances under closed loop A/F control can be detected by appropriate mathematical comparison with data compiled from experimentally predetermined values for patterns T1 and T2.
Reference values for patterns T1 and T2 are established on a balanced (i.e., all cylinders initially at stoichiometric A/F or other known A/F) three-cylinder engine by operating the engine with calibrated fuel injectors to intentionally successively impose the two patterns at the desired fuel-rich or fuel-lean levels. This calibration process is conducted at selected representative operational speeds and loads for the engine over a sufficient number of engine cycles to obtain the corresponding O2 sensor output at successive crankshaft positions. In other embodiments of the invention, a wide-range A/F sensor or a torque sensor is used. For example, at each engine speed and load, pattern T1 could be produced by a lean imbalance of +10% of stoichiometric A/F in cylinder #1, a rich imbalance of xe2x88x9210% of the stoichiometric A/F in cylinder #3 while cylinder #2 is operated at the stoichiometric A/F. Then, imbalances of like magnitude could be imposed in accordance with the T2 pattern. Assuming 60 available crankshaft position signals over two crankshaft revolutions (i.e., one engine fueling cycle), oxygen sensor data would be collected by the PCM at each 12xc2x0 of crankshaft revolution.
The data from O2 (or wide-range A/F or crankshaft torque) sensor for each template T1 and T2, at engine speed (rpm) and load (represented by manifold absolute pressure, MAP, or manifold air flow, MAF), is subjected to discrete Fourier transform (DFT) to determine its frequency spectrum. The discrete spectrum is in terms of phase and magnitude information at various frequencies related to the base engine speed and its higher harmonics. This information, together with interpolated data or suitable analytical equations, is stored in PCM table lookups for reference by the PCM during the cylinder fueling imbalance detection phase. In this case of a bank of three cylinders, the DFT vectors for templates T1 and T2 will roughly have a phase separation of 120xc2x0.
Having established reference data for the transformed templates, fuel imbalances in the operating engine can then be detected and corrected as necessary. To the extent that cylinder to cylinder imbalances in fuel injection are due to injector delivery variations, it is expected that such imbalances will follow a regular pattern, and once detected, an appropriate correction may remain effective until further usage of the injectors changes the imbalance. Accordingly, the detection and correction parts of this invention may not have to be run continually. However, as will be seen, they can also be run as frequently as required by the PCM due to speed of convergence and computational efficiency.
The detection process is initiated by the PCM and includes collecting and storing oxygen sensor data at successive crank angle signals over a few engine cycles. One complete fueling cycle providing, for example, 60 data points may be suitable. But it will usually be preferred to collect data over several cycles. This data is subjected to the same Fourier transformation process to obtain the phase and magnitude representing a single imbalance vector.
The detected fuel imbalance vector is mathematically decomposed to determine the respective contributions of the two reference vectors T1 and T2 in the total vector of imbalances measured. In other words, the coordinates of the imbalance vector in terms of the phase angles of the reference vectors and the proportion of their respective magnitudes are determined by known mathematical practices. The conversion of the imbalance vector into two component vectors permits the correction for the fueling imbalances by the PCM. The PCM determines the xe2x80x9coppositexe2x80x9d of the two components of imbalances vectors, i.e., vectors that have the same magnitude but are of 180xc2x0 phase difference, and calculates the fueling corrections that must thereafter be applied to each fuel injector to correct the fuel imbalances otherwise present in the respective cylinders. These fuel injector on-time corrections are applied cycle after cycle until the detected level of imbalances is brought below a given threshold.
As stated, the subject process may be used in response to the signals from a current production exhaust oxygen sensor, a wide-range exhaust A/F sensor, a crankshaft torque sensor or other suitable sensors used by a PCM for fuel, air or spark control in a three-cylinder engine. As is known, fuel control to individual cylinders can be accomplished by PCM control of fuel injector xe2x80x9con timexe2x80x9d. Similarly, air distribution to the three cylinder banks can be managed by PCM control of air inlet valve actuators. And, in accordance with this invention, detected imbalances in torque from individual cylinders can be corrected by PCM control of fuel or air delivery or spark timing with respect to each cylinder.
In the above-described reference templates, stoichiometric A/F, generally about 14.7 for current commercial gasolines, was used as the mean A/F value because of the wide practice of operating engines at about stoichiometric A/F for best operation of current exhaust catalytic converters. However, if it is desired to operate the engine slightly fuel rich, e.g., A/F=about 10 to 14.7, the mean value for the templates would be a selected value in this range. Similarly, where it is desired to operate in a fuel lean mode, e.g., A/F=about 14.7 to 60, a mean template value in the lean range would be used.
Other objects and advantages of the invention will become apparent from a description of embodiments of the invention which follow.